The present invention generally relates to semiconductor laser apparatuses and a method of producing them and more particularly, to a semiconductor laser apparatus having a semiconductor laser chip die-bonded to a bonding surface with a conductive die-bonding paste and a method of producing it.
As disclosed in Japanese Patent Application Laid-Open No. 6-37403, in conventional semiconductor laser apparatuses, a semiconductor laser chip is die-bonded to a predetermined position of a bonding surface, such as of a lead frame, a stem, or a sub-mount disposed on the stem, through a metal soldering material such as In, Pb/Sn (solder), Au/Sn or the like.
FIG. 5 shows a first conventional semiconductor laser apparatus in a state that a semiconductor laser chip 50 is bonded in a predetermined position of a die-bonding surface 51a of a sub-mount 51 with a metal soldering material 52. The metal soldering material 52 is a solid at the room temperature, and is deposited to the bonding surface in the predetermined bonding position by evaporation or the like. After the semiconductor laser chip 50 is placed on the metal soldering material 52, the metal soldering material 52 is heated to 150° C. or higher to melt. At this time, the semiconductor laser chip 50 is immobilized with a bonding collet or the like (not shown). Finally, the metal soldering material 52 is cooled to harden or set. Thus, the semiconductor laser chip 50 is die-bonded to the predetermined position of the die-bonding surface 51a. In FIG. 5, reference numeral 53 denotes a main-discharge-side light-emitting point of the semiconductor laser chip 50, reference numeral 54 denotes a monitoring sub-discharge-side light-emitting point of the semiconductor laser chip 50, and reference numeral 55 denotes an emission light axis of the semiconductor laser chip 50 connecting the main-discharge-side light-emitting point and the sub-discharge-side light-emitting point to each other.
In the method of producing the first conventional semiconductor laser apparatus shown in FIG. 5, the melting point of the metal soldering material 52 is high. Thus, the heating/cooling cycle takes much time and thus it takes long to produce the semiconductor laser apparatus. Further, the hardened metal soldering material 52 is thinner than 0.01 mm. Thus, if this semiconductor laser apparatus is adopted for an optical pick-up using a three-beam scheme which is a dominant tracking control method, the following problem occurs. Of the three beams which have returned from an optical disk, one auxiliary beam is regularly reflected off a discharge surface of the semiconductor laser chip 50 back to the optical disk, and is then incident on a signal detection photodiode to generate noise.
As a measure of overcoming the problem that it takes long to make the semiconductor laser apparatus, there is proposed a method (hereinafter referred to as second conventional art) of making a semiconductor laser apparatus. In the method, the semiconductor laser chip is die-bonded to a bonding surface with a conductive die-bonding paste (conductive adhesive agent) instead of the metal soldering material. The conductive die-bonding paste contains resin and a conductive filler such as silver flakes. It is possible to lower the hardening temperature of the conductive die-bonding paste to about 100° C., depending on the resin of the paste. Accordingly, the heating/cooling cycle becomes short. Thus it is possible to reduce the time required to produce the semiconductor laser apparatus.
FIG. 6 shows a semiconductor laser apparatus formed by die-bonding a semiconductor laser chip to a die-bonding surface with a conductive die-bonding paste. In FIG. 6, parts similar to or same as the parts shown in FIG. 5 are denoted by the same reference numerals as in FIG. 5. Reference numeral 56 denotes a conductive die-bonding paste.
In the method of producing the semiconductor laser apparatus according to the second conventional art, when the proportion of the conductive filler is increased to reduce the electric resistance of the conductive die-bonding paste 56, the viscosity of the conductive die-bonding paste 56 becomes high. Consequently, when a semiconductor laser chip 50 is placed on the conductive die-bonding paste 56, the conductive die-bonding paste 56 swells and adheres to the end surfaces and side surfaces of the semiconductor laser chip 50, and blocks a main-discharge-side light-emitting point 53 and/or a monitoring sub-light-emitting point 54. This will be concretely described below with reference to FIGS. 7A and 7B.
Referring to FIG. 7A, a predetermined slight amount of the conductive die-bonding paste 56, which has been discharged from a dispenser (not shown), is on a tip of a syringe needle 57. With a downward movement of the tip of the syringe needle 57 in a descending direction 58A, the conductive die-bonding paste 56 is placed in a predetermined position of a die-bonding surface 51a of a sub-mount 51. Then, with the tip of the syringe needle 57 moved in an ascending direction 58B as shown in FIG. 7B, the conductive die-bonding paste 56 is applied to a predetermined part of the die-bonding surface 51a of the sub-mount 51.
Then, as shown in FIG. 6, the semiconductor laser chip 50 is placed on the conductive die-bonding paste 56 applied to the die-bonding surface 51a of the sub-mount 51. The size of the lower surface of the semiconductor laser chip 50 is about 0.2 mm×0.2 mm, and the light-emitting point is located at about 0.05 mm from the lower surface of the semiconductor laser chip 50. That is, the light-emitting point is at a level higher than the semiconductor laser chip mounting surface 51a by about 0.05 mm. On the other hand, from the viewpoint of reliably applying the conductive die-bonding paste 56 to the die-bonding surface 51a, it is impossible to make the diameter of the tip of the syringe needle 57 smaller than about 0.3 mm. Consequently, the application area of the conductive die-bonding paste 56 is wider than the size (area of the lower surface) of the semiconductor laser chip 50, and the thickness of the conductive die-bonding paste 56 frequently exceeds 0.05 mm. Accordingly, as shown in FIG. 6, the conductive die-bonding paste 56 swells along the end surfaces and side surfaces of the semiconductor laser chip 50 mounted thereon. The end surfaces have the main discharge-side light-emitting point 53 and the monitoring light-emitting point 54, respectively. Thus, if the conductive die-bonding paste 56 is heated and cooled to harden in the above state, it follows that the conductive die-bonding paste 56 blocks the main-discharge-side light-emitting point 53 and the monitoring light-emitting point 54.